Justification:
This species is listed as Endangered because it has a very small population and range, and it is now restricted to a single island where it is rare, localised and declining owing to disturbance and habitat degradation. Measures are urgently needed to regulate access to important breeding valleys and the collection of bamboo to slow its rate of decline.

Acrocephalus caffer is found on Tahiti in the Society Islands, French Polynesia, having probably formerly occupied all the high islands in the group. The subspecies longirostris is probably extinct on Moorea although there have been at least two unconfirmed reports since 2000 (A. Gouni in litt. 2007, Cibois et al. 2008, A. Gouni in litt. 2011). The subspecies musae and garretti both went extinct on the islands of Raiatea and Huahine respectively, in the late 1800s or early 1900s. The species is absent from the Tahiti peninsula (Tahiti Iti) and has been rare and local throughout the 20th century, being recorded in six valleys during the period 1920-1923 (out of 14 visited) and 12 during 1986-1991 (out of 39), and estimated to number a few hundred individuals (Monnet et al. 1993b).

The population is estimated to number a few hundred individuals by Monnet et al. (1993b). It is placed in the band 250-999 mature individuals, equating to 375-1,499 individuals in total, rounded here to 350-1,500 individuals.

It occurs in bamboo thickets and second growth forests in river valleys and hillsides to 1,700 m. It feeds on insects but also takes lizards, small fish, crayfish, snails and nectar (Pratt et al. 1987, Thibault 1988). It is thought to breed exclusively in bamboo thickets (P. Raust in litt. 2007).

The development of hydro-electricity opened up the interior of the island with new roads and tracks (P. Raust in litt. 2007). This increased access has lead to a considerable increase in the exploitation of bamboo as well as invasion by the neotropical weed Miconia and an increase in tourists in four-wheel drive vehicles. These factors have modified the habitat considerably, and most worryingly have caused a loss of breeding habitat, as well as causing disturbance to birds (P. Raust in litt. 1999, P. Raust in litt. 2007). The introduction of feral cats Felis catus, rats Rattus spp. (A. Gouni in litt. 2012) and many alien bird species, including the aggressive Common Myna Acridotheres tristis, may also contribute to its rarity (Thibault 1988, Seitre and Seitre 1991).

Conservation Actions ProposedResurvey the species in order to establish trends (C. Blainvillain verbally 2000). Control the exploitation of bamboo. Monitor numbers in easily accessible key sites. Investigate the severity of the threat caused by A. tristis and other predators. Consider control of predators at sites known to be important to the species. Protect important sites from habitat clearance through road and dam building and degradation by off-road vehicles.